Various photosensitive materials have been practically used in electrophotographic systems including the steps of charging, light exposure and development (see, e.g., U.S. Pat. No. 2,297,619). For example, it is possible to directly form, on a proper conductive base plate, an organic photoconductive material by coating or vapor deposition. In another system the aforesaid material is provided on the base plate together with a proper organic binder. It is also possible to have a binder layer formed on the base plate having dispersed therein an inorganic photoconductive material such as ZnO, CdO or TiO.sub.2. In yet another system a layer of amorphous selenium or an alloy thereof is formed on the base plate by vapor deposition. It is also possible to have two or more of the above-described photoconductive layers laminated on a base plate (see, e.g., U.S. Pat. Nos. 3,850,630 and 4,175,960). For obtaining good electric and optical properties as well as mechanical properties or further improving and stabilizing or, as the case may be, improving the characteristics for processes such as development in these photosensitive materials, it has been proposed to provide a surface layer on the surface of the photosensitive material. One of these surface layers is a so-called protective layer. For example, a thin resin layer is formed on the surface of a photoconductive layer and latent images are formed by performing charging and image exposure (Carlson process). However, the use of photosensitive material having such a protective layer frequently results in the occurrence of high residual potential and large cycle increase of residual potential, providing copies having deteriorated image quality with fogging. The occurrence of high residual potential and large cycle increase of residual potential can be considerably improved by reducing the thickness of the protective layer below 1.mu., but in this case, the layer is liable to be separated. Therefore, such a photosensitive material cannot be used for a long period of time. Another example of the surface layer is a so-called insulating layer, i.e., a resin layer having a high electric resistance, wherein latent images are formed by a specific process including an electricity eliminating process (e.g., see U.S. Pat. No. 3,041,167). However, photosensitive material having an insulating layer requires a specific latent image-forming process. For example, such a material requires at least two charging steps. Therefore, complicated apparatus is needed.
This invention relates to a photosensitive material having a former type protective layer which can form latent images by a so-called Carlson process without requiring any specific latent image-forming process.
Previously, the assignee proposed the use of a low-resistant protective layer for overcoming the abovementioned difficulties (see, Japanese Patent Application Nos. 42,118/'79; 65,671/'79; 65,672/'79 and 65,673/'79). By forming such a low-resistant protective layer, the thickness of the protective layer can be increased to 10-20.mu. and the occurrence of high residual potential and large cycle increase can be prevented. However, it has been found that in these processes, the charging characteristics of the entire photosensitive material may be reduced. Therefore, it is not possible to obtain images having sufficient contrast. This tendency is particularly remarkable with photosensitive materials having a high-sensitive photoconductive layer.